I've just ordered a Mercury On-the-go, triple interface, 500GB. I haven't got it yet, so can't tell you how good it is, but although it won't win any beauty contests, it's had good reviews.
Any external hard drive will work with PCs or Macs, as long as the connectors are there (Firewire, USB, etc.) It doesn't matter how the drive is formatted out of the box, since you can re-format any way you like. Formatting in HFS (Mac OS Extended) or FAT32 or NTFS-3G can be done with the Mac OS X Disk Utility.
FAT32
Read/Write FAT32 from both native Windows and native Mac OS X.
Just format your drive and it's compatible with your Mac. If your Mac does not recognize the drive it asks if you want to initialize it and off you go-its done in a just a few minutes.
Get a FW800 if it works with your Mac. They are faster by far. I have more than one, but the LaCie Rugged 500GB FW800 7200rpm is TERRIFIC! and portable. Even the Apple store Genius bar uses this type near me.